Opium production in Afghanistan has dropped by just 2 percent this year, despite a major clampdown on poppy farmers that sharply reduced the amount of land used to grow opium poppies, the U.N. anti-drug chief said Monday.

The land under cultivation was reduced by 21 percent, but heavy rains helped produce a bumper crop of 4,100 tons of opium, said Antonio Maria Costa, executive director of the U.N. Office on Drugs and Crime.

Afghanistan is still estimated to produce 87 percent of the world's supply of opium and its derivative, heroin, Costa said.

He predicted it would take 20 years to eradicate cultivation of opium poppies, a mainstay of Afghan farmers despite government warnings and the destruction of some crops.

"We see a significant improvement in the amount of land cultivated in Afghanistan," Costa said, but "heavy rainfall, snowfall and no infestation of crops resulted in a very significant increase in productivity."

A report by the U.N. agency said the total amount of land being used to grow poppies dropped from 323,570 acres in 2004 to 256,880 acres this year. But the jump in crop yield was 22 percent.

The United States, Britain and other countries have poured hundreds of millions of dollars into an anti-drug campaign in Afghanistan after opium and heroin production ballooned in recent years.

Costa praised President Hamid Karzai for trying to eradicate drug production, but said some officials were involved in the drug trade and should be removed.